The present invention relates to drip emitters for use with drip irrigation hose and, in particular, it concerns a bi-component drip emitter for attachment to an inner surface of a wall of an irrigation hose.
Drip emitters for use in irrigation hoses include a flow restriction to reduce flow rates to the desired level. Examples of flow restrictions include, but are not limited to, laminar flow restrictions, and patterns of meandering fine flow passageways referred to as labyrinths.
In some cases, elastomer materials are employed as part of a flow restriction designed to provide a variable flow restriction geometry. The variable geometry is designed to be responsive to the fluid pressure within the supply hose so as to at least partially compensate for flow rate variations due to changes in the pressure within the hose. An example of such an arrangement is my U.S. Pat. No. 5,400,973 in which a flat elastomer diaphragm is deformed by pressure within the hose to sequentially close clearances across the top of a sequence of baffles forming a labyrinth.
The above approach is highly advantageous, providing pressure compensated regulation of the drip outlet flow rate. However, the use of a separate elastomer diaphragm dictates a two-part, or more often three-part, design, requiring an assembly step during production and thus increasing production costs.
As an alternative approach, an elastomer may be employed as a bulk material for production of a single piece drip emitter with pressure compensation. Examples of such devices may be found in my U.S. Pat. No. 6,886,761, which employs an elastomer labyrinth with varying height baffles such that pressure increase in the hose sequentially closes a bypass channel formed between the baffles and the wall of the hose. A further example is my published Patent Application No. WO 2008/035335 in which an elastomer labyrinth is formed from baffles with an upright upper wall portion and a thickened root portion which forms a sloped transition region between the upper wall portion and the base. In this case, when the fluid pressure within the irrigation hose increases, the sloped transition region becomes progressively flattened to define an increased turbulence flow path geometry, thereby regulating the flow rate through the drip emitter.
In both of these cases, the elastomer component is formed by an extrusion process, or co-extrusion. The resulting structures have many advantages, but require a relatively large proportion of elastomer material, and are limited to the use of certain types of elastomer which provide sufficient structural support and which can be effectively bonded to the inner surface of an irrigation hose. Furthermore, production by extrusion techniques inherently limits the range of shapes that can be produced, and provides relatively low manufacturing precision.
Finally, reference is made to my U.S. Pat. No. 5,203,503 which discloses a bi-component drip emitter produced by a bi-component injection molding technique. The bi-component injection molding technique perform two-stage injection with relative movement of the mold parts between stages, resulting in a unitary structure in which an elastomer component defining a fine laminar flow restriction is implanted within a rigid polymer device. This requires a small quantity of elastomeric material to provide a variable geometry flow restriction while the rest of the drip emitter structure is formed from rigid polymer material, thereby reducing production costs and providing a structure well suited for bonding to the inner surface of an irrigation hose.
While promising significant advantages as mentioned, practical implementation of a bi-component drip emitter by bi-component molding presents various challenges. Specifically, the device disclosed in my U.S. Pat. No. 5,203,503 typically fails to achieve reliable flow rate compensation due to the high sensitivity of the laminar flow restriction to variations in geometry (requiring ten times more precision than the turbulent variable geometry labyrinth options described below). Performance of the variable flow restriction is also typically very sensitive to variations in positioning of the drip emitter relative to the inner surface of the hose wall due to variations in the welding attachment process.
Finally, thermoset elastomers such as silicone have various potential advantages over other elastomer materials for production of compensated drip emitters, for example, exhibiting high resistance to creep. However, bi-component molding techniques are challenging to implement for devices which combine thermoset and thermoplastic components, and are particularly problematic for production of drip emitters due to the failure of silicone to bond firmly with adjacent rigid polymer material.
There is therefore a need for a bi-component drip emitter which would provide improved flow compensation, which would render the device less sensitive to variations in position of the drip emitter relative to the inner surface of the hose wall, and/or which would provide effective retention of a component folioed from silicone.